1. Field of Invention
The present invention relates generally to printed circuit boards and electronic components used thereon and, more particularly, to passive electronic components and printed circuit boards including passive components.
2. Discussion of Related Art
Passive components, such as resistors, capacitors and inductors, are commonly provided on printed circuit boards as surface mount (SMT) components. Referring to FIG. 1, there is illustrated one example of a standard surface mount resistor 100. The resistor 100 has a package body 102 with two solderable pads 104 for connecting the resistor to a printed circuit board. Surface mount passive components are generally fairly small. For example, a standard surface mount resistor, such as that illustrated in FIG. 1, may be approximately rectangular with a length, L, of about 0.040 inches (±0.002 inches), a height, h, of about 0.018 inches (±0.002 inches), and a width, w, of about 0.020 inches (±0.002 inches). However, despite the already small size of these components, the drive toward smaller and smaller electronic devices (e.g., cellphones or cameras), and thus, smaller and more densely populated printed circuit boards, makes placement of passive surface mount components on printed circuit boards challenging.
In particular, one common type of printed circuit board package is the ball grid array (BGA). A BGA uses an array of solder balls attached to its solderable pads on one side; it uses the solder balls to connect to the circuit board or other substrates. Generally BGAs have very limited space on which to place passive components, such as resistors. This limited space creates a challenge for optimal component placement. In addition, many printed circuit boards are now multi-layer boards and therefore, vias are used to connect surface mount components to signal traces on other board layers as well as to the solder balls of the BGA. For example, referring to FIG. 2, there is illustrated an example of a BGA circuit board 106 with solder balls 108 making up the BGA. Vias 112a and 112b, and traces 110a and 110b, are used to connect the surface mount resistor 100 to the BGA, as shown. The trace 110b forms an extra stub length, which can add undesirable inductance, but is required to connect the surface mount resistor 100 to the solder ball 108 of the BGA.
FIG. 3 illustrates an example of a vertical stub caused by a via connecting a surface mount resistor to inner signal layers of a multi-layer board. A surface mount resistor 100 is mounted on layer one (which is the top or bottom of the circuit board) of a multi-layer printed circuit board and connected (by its solderable terminals 104) to a signal trace 110. A via 112 is used to connect the surface mount resistor 100 to signal traces 116 on other layers. In the illustrated example, a via pad 114 couples the via 112 to a signal trace 116a on layer two, thereby forming a series connection of the trace 110, resistor 100 and trace 116a. The remaining length of the via 112 from layer three to the bottom via pad 118 on layer six is an extra vertical stub length that adds additional inductance and could act as an antenna.
An alternative to surface mount passive components are buried passive components. With buried passive components, for example, resistors, the resistor is screened onto an inner layer of the circuit board, generally in a rectangular area, as shown in FIG. 4. In FIG. 4, a buried resistor 120 is screened onto a signal layer 122 of a circuit board and connected to traces 116 and vias 112. Although the buried component 120 may be placed on a inner layer, thereby possibly eliminating a via needed to connect the component 120 from the top or bottom of the board to a signal trace on an inner layer, vias are still needed to form connections between the signal traces on different layers. Thus, the presence of the buried component 120 on the signal layer 122 actually uses up valuable via and signal trace space on the inner layers of the board.